block: Add BlockErrorKind classification enum

Add a small, stable enum that classifies block errors into
broad categories - I/O, invalid format, unsupported feature,
corrupt image, out of bounds, not found, overflow. Callers
match on this for control flow rather than on format specific
error variants.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/error.rs

@@ -15,3 +15,42 @@
 //!         |-- VhdError / RawError / ...
 //!         +-- io::Error / etc.
 //! ```
+
+use std::fmt::{self, Display, Formatter};
+
+/// Small, stable classification of block errors.
+///
+/// Callers match on this for control flow. Adding new format specific
+/// errors does not require new variants here.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum BlockErrorKind {
+    /// An underlying I/O operation failed.
+    Io,
+    /// The disk image format is structurally invalid.
+    InvalidFormat,
+    /// The disk image requires a feature that is not implemented.
+    UnsupportedFeature,
+    /// The image is marked or detected as corrupt.
+    CorruptImage,
+    /// An address, offset, or index is outside the valid range.
+    OutOfBounds,
+    /// A file or required internal structure could not be found.
+    NotFound,
+    /// An internal counter or limit was exceeded.
+    Overflow,
+}
+
+impl Display for BlockErrorKind {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Io => write!(f, "I/O error"),
+            Self::InvalidFormat => write!(f, "invalid format"),
+            Self::UnsupportedFeature => write!(f, "unsupported feature"),
+            Self::CorruptImage => write!(f, "corrupt image"),
+            Self::OutOfBounds => write!(f, "out of bounds"),
+            Self::NotFound => write!(f, "not found"),
+            Self::Overflow => write!(f, "overflow"),
+        }
+    }
+}